What is "file-system"
File systems (or filesystem)
A file system is the specification of how files on a computer should be logically stored, named, and organized. File systems presents users with a readable data organization format on the computer, where each filing cabinet is a discrete data unit.
File systems have only one logical format. They do not necessarily reflect the way data is physically stored on a computer disk drive or other storage device. Since the computer actually stores data in bits, the data on a storage device does not have any structure, consisting of nothing more than a series of 1s and 0s.
File systems therefore give structure and meaning to this data, giving - at the very least - an arbitrary collection of bits, a filename and keeping a record of the physical location of files on the storage device.
Common elements
In addition to file names, normal file systems also describe and maintain directories and file attributes, which serve as a means to better structure data organization to give specific purposes, security features, or functionality to files.
File systems usually store this information in table format in a specific region of the storage device. Each file is listed in this table with its location, length and other clearly stated properties. An operating system that understands file system format can then read this table to access a file to edit, rename, delete, move or change any of its attributes.
For further reading
Filesystem on Wikipedia